GRAs are used, for example, in aircraft for actuating flaps, slats, and other aerodynamic control surfaces. GRAs typically incorporate a torque limiter for limiting transmission of torque between an input shaft and an output shaft of the GRA in the event of a malfunction. Conventional torque limiting devices include a disc brake pack having multiple brake discs utilizing frictional contact between adjacent discs for limitation of torque transmission. Such torque limiting devices have several inherent problems. Because the friction coefficient is very sensitive to lubrication, changes in the lubrication environment can cause the friction coefficient to drop below a critical value required to provide a positive torque limit. This can cause the torque limiter to exceed the maximum torque limit setting. If too little lubrication is present in the disc brake pack and moisture is present, the disc brake pack can freeze up, causing nuisance lock-ups. When adequate lubrication is provided to the disc brake pack, considerable viscous drag is present. The viscous drag is not a problem as long as it is accurately predicted and accounted for in the torque limiter setting and power control unit (“PCU”) sizing, however, such viscous drag causes inefficiency in the system and higher limit loads on components downstream of the torque limiter.
Known torque limiting mechanisms respond to input torque to the GRA rather than GRA output torque. Consequently, the lock-up torque limit setting must be significantly higher than the maximum operating torque of the GRA, and therefore the GRA is designed with a relatively large limit output torque. As a result, each GRA has a greater weight associated therewith, and structure downstream from the GRA is increased. Given that an aircraft may have many GRAs, for example thirty or more, a cumulative weight cost is imposed on the aircraft design.
There is a need for a torque limiter that solves the problems described above.